Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

422
FXUS63 KLOT 211106
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
606 AM CDT Sun Jun 21 2026

.KEY MESSAGES...

- A storm system will bring widespread soaking rainfall and
  possibly a few storms to the area today into tonight. A
  conditional level 1/5 risk for severe weather is present for
  areas well south of I-80.

- A corridor of locally heavier rainfall rates may occur somewhere
  in our area today and lead to flooding. The greatest chances
  for excessive rainfall are present near and south of I-88, where
  there is a level 2/4 risk for flooding.

- A heightened risk for rip currents will exist at southern Lake
  Michigan beaches from this evening through Monday night.

&&

.DISCUSSION...
Issued at 342 AM CDT Sun Jun 21 2026

Through Tonight:

GOES satellite and WSR-88D radar mosaic imagery this morning
depict an expansive convective complex across the central Plains
working its way towards the east and southeast. An MCV is
evident in central Kansas at the time of this writing, displaced
to the south of the parent shortwave that helped kickstart this
convection yesterday afternoon as it ejected eastward out of
the central Rockies. As this MCV, shortwave, and an associated
surface low continue to progress eastward today, an expansive
precipitation shield will spread over our forecast area from
west to east from the mid-late morning through this afternoon,
bringing a widespread soaking rainfall to the area that should
last for several hours. This is an aspect of high confidence
with our forecast. Two forecast aspects that remain of lower
confidence, however, are the potential for flooding and
thunderstorms in our area today as this system moves through the
region.

One feature that will play a key role in whether either of
these threats are realized in our area today is a warm front
that was analyzed to be present from near Kansas City to just
north of Jackson, TN at 0800Z. This warm front will effectively
serve as the northward terminus of where a threat for severe
weather will be present today and will also help dictate how far
north the threat for lightning and convectively-enhanced
rainfall rates will trek. The latest CAM guidance, for what it`s
worth (which probably isn`t a whole lot as it, unsurprisingly,
does not have a good handle on the ongoing convection),
generally favors the warm front lifting northward into central
Illinois and Indiana this afternoon and possibly as far north as
our southernmost tier of counties. These CAMs are modeling this
outcome under the assumption that the ongoing central Plains
MCS will slow down and/or decay through the remainder of the
night and into the morning, affording the warm front enough time
to lift this far north before having its progress stalled by
the incoming convection and rain.

While the MCS has notably weakened over the past few hours, the
embedded MCV still appears to be well-defined and is generating
new convective cells out ahead of it, and there is a reservoir
of untapped 1000-2000 J/kg of MUCAPE out ahead of the complex in
southern Missouri, so still have some doubts over whether this
MCS will lose enough steam for the warm front to be able to make
it as far north as the aforementioned CAM solutions are
suggesting. Nevertheless, if the warm front were indeed to make
it as far north as our southern CWA, then rich low-level
moisture paired with strong low-level and effective shear values
resulting from 50+ kt mid-level flow and backed surface winds
near the warm front would yield a threat for low centroid
supercell structures or bowing segments capable of producing
tornadoes and damaging winds in our far southern counties today
(in the area more or less delineated in SPC`s Day 1 Convective
Outlook). That said, the more likely scenario at this time is
that the warm front and the attendant severe weather threat will
remain south to potentially well south of our forecast area as
a result of the incoming central Plains convection arriving at
our longitude faster than indicated in CAM guidance and
accordingly forcing the warm front to remain farther south than
indicated in these models.

Even if the warm front does not quite reach our area, there are
still indications that a layer of meager elevated instability
could claw as far north as the I-80 or I-88 corridor late this
afternoon and evening and pose some lower-end threat for
isolated lightning strikes in our area during this late day time
frame while the main show remains to our south. Additionally, it
still appears that the main synoptic surface low will track
across our southern counties or just to our south across central
Illinois and Indiana. Several models have been depicting
notable frontogenetical forcing along the northern flank of the
surface low inducing a band of precipitation featuring enhanced
rainfall rates. Where this band of precipitation sets up and
just how torrential the rainfall ends up being within it will
depend on the exact strength and track of the surface low,
which, due to the extensiveness of the upstream convection,
remains extremely difficult to pin down at this time.

In a scenario with a stronger surface low and associated
deformation/f-gen band, rainfall rates could be efficient enough
amidst 1.5-2" precipitable water values to cause flooding
somewhere in or very near our forecast area. The latest flash
flood guidance (FFG) values from the North Central River
Forecast Center are generally around 1.5-2" for 3 hours and
2-2.5" inches for 6 hours, and while there are a few exceptions
(e.g. lower values for our more heavily urbanized and flood-
prone areas), those are generally the rainfall thresholds that
we believe could start to cause some more notable hydrologic
concerns to arise if they are exceeded with the specified time
interval(s). Confidence in whether these thresholds will be
exceeded in our forecast area today and where was too low to
warrant the issuance of a Flood Watch with the overnight
forecast package, but couldn`t rule out the need for a short-
fused Flood Watch issuance by the day shift if the forecast
picture clears itself up more later this morning. Either way,
area creeks, streams, and rivers will rise as a result of this
rainfall, and it`s not out of the question that some additional
river flooding could occur somewhere in the near future as a
result of today`s rainfall.


Monday through Saturday:

Some shower activity from Sunday`s storm system may linger
around through Monday morning, but should clear our area by the
early afternoon. During the afternoon, couldn`t rule out
diurnal cumulus growing deep enough to support a few spits of
rain wherever surface convergence/confluence is maximized, but
otherwise, expect a dry day on Monday with highs in the mid 60s
(near Lake Michigan) to the mid 70s (well inland) as surface
high pressure begins to build into the region. Buoyed by breezy
onshore flow, high waves that will develop across southern Lake
Michigan today will persist into Monday and continue to create a
heightened threat of rip currents at area beaches through
Monday night. Winds and waves will subside on Tuesday as the
high pressure center passes close by, while daytime temperature
readings will likely only be a touch milder compared to Monday.

Late Tuesday night into Wednesday, global ensemble and
deterministic guidance remains in good agreement that an upper-
level trough will slide into the Upper Midwest, likely
accompanied by a low pressure system at the surface. Moisture
profiles should be plenty adequate to support rain showers
across the region as this system passes by, and while lapse
rates look to be lackluster on the whole, deterministic GFS and
ECMWF soundings both depict CAPE profiles with equilibrium
levels solidly above -20C that should be favorable for
thunderstorms to occur in the area as well.

Ensemble consistency breaks down over the latter half of the
week into the weekend, but still broadly suggests that a few
follow-up shortwave troughs could traverse the region in the
days following Wednesday. Thus, additional showery and perhaps
stormy periods could be seen going into the weekend, but
confidence in the specific details of that potential remains
low at this time. Confidence is higher in near to below normal
temperatures persisting through the entirety of this upcoming
week, though ensemble guidance does suggest that some warmer
temperatures could be on the horizon just beyond the end time
of our current 7-day forecast period.

Ogorek

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 606 AM CDT Sun Jun 21 2026

Key Messages:

- Rain develops eastward across the terminal airspace late this
  morning through early afternoon.

- Steadiest rain with periods of low MVFR to IFR CIGs and VSBYs
  expected late this afternoon into this evening.

- Light and variable winds to start the day quickly settle east-
  southeastward by late morning, then turn breezy from the east-
  northeast late this afternoon and evening.

Our next weather system will move into the area today. As it
does, expect rain to overspread the area by midday/early
afternoon. The more persistent and heavier rainfall is likely
to impact the terminals late this afternoon into this evening.
Lower MVFR VSBYs and low MVFR to IFR CIGS are likely to
accompany some of the more persistent rain this evening. There
is also a non-zero chance of a couple of isolated lightning
strikes with this activity, though the better chances for this
looks to be south of the terminals. Otherwise, he rain will
gradually taper off late this evening and overnight. In spite of
this, low cloud cover may persist through early Monday morning
before improving.

Light and variable winds early this morning will settle into an
east-southeasterly direction later this morning as the system
approaches. Winds will then turn breezy from the east, and
eventually the northeast late in the day and this evening as
the surface low tracks eastward across central IL and IN.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from this evening through late Monday
     night for ILZ006-ILZ103.

     Beach Hazards Statement from this evening through late Monday
     night for ILZ104.

IN...Beach Hazards Statement from this evening through late Monday
     night for INZ001-INZ002.

LM...Small Craft Advisory from 4 PM this afternoon to 1 AM CDT
     Tuesday for the IL nearshore waters.

     Small Craft Advisory from 4 PM this afternoon to 4 AM CDT
     Tuesday for the IN nearshore waters.

&&

$$

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